Polymer compositions are often considered for use as electrical insulating materials, because they typically do not readily conduct electrical current and are generally rather inexpensive relative to other known insulating materials. A number of known polymer compositions are sufficiently durable and heat resistant to provide at least some electrical insulating utility, but many such polymer compositions are problematic due to the accumulation of electrostatic charge on the surface of the material.
The accumulation of surface charge on an insulating material is undesirable for various reasons. Such materials sometimes discharge very quickly, and this can damage electronic components, or cause fires or explosions, depending upon the environment. Sudden static discharge can also be an annoyance to those using the material.
Even where sudden static discharge is not a problem, dust will typically be attracted to and will accumulate on materials carrying a static charge. Furthermore, the static charge can interfere with sensitive electronic components or devices and the like.
Consequently, a need exists for electrostatic dissipating ("ESD") polymeric compositions having an appropriate resistivity. That is, these compositions must have sufficient resistivity to cause the "bleeding off" or dissipation of any occurring static charge. The resistivity must not be so low as to allow the charge to move too quickly through the material, thereby causing an arc or spark. On the other hand, the resistivity must not be so great as to cause the charge to build up to such a high level as to ultimately cause a sudden discharge (spark or arc).
Resistivity can be further defined as involving surface resistivity and volume resistivity. If the volume resistivity is in an appropriate range, an alternative pathway is provided through which a charge can dissipate. However, many conventional electrostatic dissipative materials, particularly materials comprising low molecular weight antistatic agents, provide electrostatic dissipative properties primarily by means of surface resistivity. As a result, surface resistivity is typically the primary focus for electrostatic dissipating materials.
Surface resistivity is an electrical resistance measurement (typically measured in Ohms per square) taken at the surface of a material at room temperature. Where the surface resistivity is less than or equal to about 10.sup.5, the composition's surface has very little insulating ability and is generally considered to be conductive. Such compositions are generally poor electrostatic dissipating polymeric materials, because the rate of bleed off is too high and sparking or arcing can occur.
Where the surface resistivity is greater than 10.sup.12, the composition's surface is generally considered to be an insulator. In certain applications, such a composition is also a poor electrostatic dissipating material, because the surface does not have the requisite amount of conductivity necessary to dissipate static charge.
Typically where the surface resistivity is about 10.sup.6 to 10.sup.12, any charge contacting the surface will readily dissipate or "decay". Further information involving the evaluation of surface resistivity can be found in American Standard Test Method D257.
Static charge decay rates measure the ability of an electrostatic dissipating (ESD) material to dissipate charge. A 90 percent decay time as used herein is measured at about 15 percent relative humidity and at ambient temperature as follows: a 5000 Volt charge is placed upon the material and the amount of time (in seconds) for the charge to dissipate to 500 Volts is measured. A 99 percent decay time is measured substantially as for the 90 percent decay time, except that the amount of time measured is for the charge to dissipate to 50 Volts.
Many electrostatic dissipating materials generally found in the art have a 90 percent decay time of greater than about 3 seconds and a 99 percent decay time of greater than about 5 seconds. However, the National Fire Protection Association standard (NFPA Code 56A) requires 0.5 seconds as an upper limit for a 90 percent decay time, and the U.S. Military Standard (MIL-81705B) requires 2.0 seconds as an upper limit for a 99 percent decay time.
Attempts have been made to coat an electrostatic dissipative material onto an insulating plastic to reduce the accumulation of static charge. Surface applications however have been problematic due to long term adhesion requirements and interference with surface properties.
Other attempts to reduce the accumulation of static charge include the addition of graphite, metals, organic semiconductors or other low molecular weight antistatic agents. However, problems have arisen relating to the processability and/or the physical properties of the resulting product.
Rigid additives, such as metal and graphite, often deteriorate the physical and mechanical properties of the plastic. Such additives can also be expensive and make processing difficult.
Conventional low molecular weight electrostatic additives typically work well only in the presence of high relative humidity. Such additives typically must bloom to the surface after blending or mixing to provide electrostatic dissipative performance, and such blooming may not always be consistent or may cause thermal stability problems or may cause physical properties to deteriorate. Such additives can also create an undesirable film or can wash away or abrade from the surface.
Low molecular weight electrostatic dissipating additives generally can be blended with polymers having a high glass transition temperature, such as rigid polyvinyl chloride (PVC), polystyrene, acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile (SAN), and styrene-maleic anhydride (SMA); however the high glass transition temperature of such plastics typically hinders subsequent migration of the electrostatic dissipating additives to the surface of the cooled parts, and such migration is typically necessary to obtain desired electrostatic properties. Blending may also require such high temperatures as to cause discoloration, instability and degradation of the material.
The following patents relate to the incorporation of high molecular weight (polymeric) electrostatic dissipating agents into plastic.
U.S. Pat. No. 4,588,773 to Federal et al discloses an electrostatic dissipating thermoplastic composition comprising a copolymer of acrylonitrile, butadiene, and styrene (ABS) and a copolymer of epihalohydrin copolymer.
U.S. Pat. No. 4,775,716 to Kipouras et al discloses an electrostatic dissipating ABS blend comprising epihalohydrin-oxirane copolymer wherein the amount of ABS is 80 percent by weight or more. The required ratio of epihalohydrin to oxirane is defined as being between about 1:19 to about 7:13 by weight.
U.S. Pat. No. 4,719,263 to Barnhouse et al discloses an electrostatic dissipating Composition comprising an epihalohydrin homopolymer or copolymer and chlorinated polyvinyl chloride, polycarbonate, polyester, epoxy, phenolics, or mixtures thereof.
Published European application no. 282,985 to Yu discloses a copolymer of epihalohydrin and ethylene oxide as an electrostatic dissipating additive. The preferred composition is defined as being at least 60 percent by weight ethylene oxide.
U.S. application Ser. No. 039,258, filing date Apr. 17, 1987, to Yu is directed to an electrostatic dissipating polymeric composition comprising an electrostatic dissipating copolymer of ethylene oxide and a comonomer selected from the group consisting of cyclic ethers, cyclic acetals, and cyclic esters. The polymeric composition can further comprise any thermoplastic, thermoplastic elastomer or elastomer.
Published European application Ser. No. 294,722 discloses the use of polymethylmethacrylate (PMMA) in blends of SAN containing epichlorohydrin copolymer.
U.S. Pat. No. 4,230,827 to Myers discloses ethylene oxide polymers as being useful as impact modifiers for PVC. The Myers patent teaches that the ethylene oxide polymer must be comprised of at least about 80 percent by weight ethylene oxide. The ethylene oxide polymer is further defined as having a viscosity average molecular weight of about 200,000 to about 10,000,000.